The present invention relates to a color separation/combination prism suitable for an image-projection apparatus which projects an image using an image-forming element such as a reflective liquid crystal panel.
A color separation/combination system of a projector using a reflective liquid crystal panel needs to perform color separation and color combination of red (R), green (G) and blue (B) and split polarized light modulated by the liquid crystal panel. Such a color separation/combination system generally has a configuration using three to four optical elements including two polarization beam splitters (PBSs) having a polarization beam splitting film which forms an angle of 45 degrees with respect to an optical axis of incident light.
Japanese Patent Laid-Open No. 2001-154152 discloses an optical system having two polarization beam splitters and two dichroic mirrors arranged on the optical axis of a reflective liquid crystal panel. It also discloses a configuration in which three polarization beam splitters and one dichroic mirror are arranged. According to these configurations, four optical elements are arranged in a positional relationship of 2×2 and optical films of the four optical elements are arranged in two directions orthogonal to each other. The four optical elements perform color separation, light analysis and color combination.
Furthermore, Japanese Patent Laid-Open No. 2002-162520 discloses a prism which singly performs color separation, color combination and polarization split. This prism is configured by arranging in a cube three polarization/dichroic beam splitting films which form an angle of 45° with respect to the optical axis of incident light. The cube has twelve sides therein. Of those sides, three sides meet at each vertex. A polarization/dichroic beam splitting film is arranged on a surface that forms an angle of 45° with respect to two surfaces contacting each of the three sides and includes that side. In this way, a prism including three 45° polarization/dichroic beam splitting films which transmit through one vertex is formed.
Reflective liquid crystal panels of RGB are respectively arranged in the vicinity of three surfaces which meet at one vertex out of the surfaces of the cube, and an entrance surface for light from an illumination system and an emergence surface for light toward a projection lens are arranged on the remaining surfaces. The entering light from the illumination system is color-separated by two polarization/dichroic beam splitting films. The light subjected to image modulation on each panel is analyzed and color-combined by two polarization/dichroic beam splitting films and guided to a projection lens.
Furthermore, Japanese Patent Laid-Open No. 2003-98315 discloses an optical system which causes light to transmit through one cross-dichroic mirror before and after a reflective liquid crystal panel. Entering light from an illumination system is color-separated into light beams of RGB colors by the cross-dichroic mirror, and the separated light beams then obliquely enter panels. The light reflected by the panel at a certain angle enters the cross-dichroic mirror again, but the light enters an area different from the area through which the entering light from the illumination system is transmitted. The color-combined RGB light is guided to a projection optical system.
However, in the optical system disclosed in Japanese Patent Laid-Open No. 2001-154152, the light reflected by each reflective liquid crystal panel is guided to the projection lens through two of the four optical elements. This is because two actions are required; light analysis using the polarization beam splitter and color combination using the dichroic mirror or polarization beam splitter. Therefore, a total of four optical elements are required, and this makes the configuration more complicated. This also extends the optical path length and causes each optical element to grow in size for securing necessary light. Moreover, the back focus of the projection lens becomes longer, which causes the projection lens itself to grow in size, too.
Furthermore, according to the prism disclosed in Japanese Patent Laid-Open No. 2002-162520, border lines produced by the beam splitting films exist in all areas of the prism and the border lines which exist in the areas facing the liquid crystal panel have an influence on an image.
Furthermore, light rays from each liquid crystal panel become light rays horizontal to a polarization/dichroic film which is different from the two polarization/dichroic beam splitting films that function as the beam splitting films for the light rays and enter the different film at a large incident angle. Thereby, the light rays are reflected at a high reflectance to cause a ghost.
Furthermore, according to the optical system disclosed in Japanese Patent Laid-Open No. 2003-98315, color-separated RGB light beams are obliquely entered into the liquid crystal panel to prevent the illumination system and the projection optical system from physically interfering with each other. Therefore, the distance from the cross-dichroic mirror to the liquid crystal panel becomes longer and the cross-dichroic mirror also grows in size. Moreover, incident light upon the reflective liquid crystal panel also has a large angle, which not only is disadvantageous from the standpoint of the incident angle characteristic of the panel but also requires the projection optical system to be a decentered optical system.